Traditionally, there are basically two processes for the production of special pulps having a high content of alpha cellulose. These include far-extended acidic bisulfite cooking, and prehydrolysis-sulfate (kraft) cooking. The former was developed at the beginning of the 20th century, and the latter in the 1930's, see e.g. Rydholm, S. E., Pulping Processes, pp. 649 to 672, Interscience Publishers, New York, 1968. The basic idea in both processes is to remove as much hemicellulose as possible from cellulose fibers in connection with delignification, so as to obtain a high content of alpha cellulose. This is essential because the various end uses of such pulps, dissolving pulp for instance, do not tolerate short-chained hemicellulose molecules with a randomly grafted molecular structure.
In the traditional sulfite process, the removal of hemicellulose takes place during the cooking, simultaneous with dissolving of the lignin. The cooking conditions in that case are highly acidic, and the temperature varies from 140.degree. C. to 150.degree. C., whereby hydrolysis is emphasized. The result, however, is always a compromise with delignification. No higher content of alpha cellulose is obtained. Another drawback is the decrease in the degree of polymerization of cellulose and yield losses, which also limit the potential for hydrolysis. Various improvements have thus been suggested, such as modification of the cooking conditions, and even a prehydrolysis step followed by an alkaline sulfite cooking stage. In spite of developments in connection with sulfite special pulp processes, the number of sulfite pulp mills in operation have decreased, and new developments have not been adopted. The main obstacle in connection with sulfite pulping processes is the complicated and costly recovery processes of the cooking chemicals, particularly of the sulfite itself.
A separate prehydrolysis step permits the desired adjustment of the hydrolysis of hemicelluloses by varying the hydrolysis conditions. In the prehydrolysis-kraft cooking process the necessary delignification is not carried out until a separate second cooking step. The prehydrolysis is carried out either as a water or steam phase prehydrolysis, or in the presence of a catalyst. In the former processes, organic acids liberated from wood during the process perform a major part of the hydrolysis, whereas in the latter processes, small amounts of mineral acid or sulfur dioxide are added to "assist" the prehydrolysis. The delignification step has been a conventional kraft cooking method, where white liquor has been added to the digester and the cooking has been carried out as a single step after removing some or none of the prehydrolysate. One of the drawbacks of this process is e.g., that the neutralized hydrolysate (free hydrolysate left in the digester, as well as immobilized hydrolysate inside the chips) causes consumption of cooking chemicals and loading of the digester.